I never tried the New Trent, but the Anker Astro 3E works quite well.
The Nook HD looks for a signaling circuit in a charger, or backup battery to see if it can draw full current. Most chargers and batteries don't have this implemented (this is why most external batteries and wall adapters will not show "charging" on the Nook HD).
The Astro 3E does include this circuit in its "Android" output port, so it charges the Nook HD well.
The main reason many USB chargers won't handle an HD/+ is that the common USB spec is for 500mA (spread among the ports on a powered hub). Some devices now provide 1A ports -- meant to drive portable hard disks.
The HD/+ wants twice that, 2A, to properly charge. A smaller supply might charge while the display is off, but very slowly.
As for the negotiation, all proper USB devices are supposed to do that (I believe the newest spec allows for dual levels to provide a charger port -- when the source device is turned off these special ports remain powered at a higher amperage, as I recall).
I just bought the same unit on ebay for $25.99 delivered that turnsouth bought, the Anker Astro 3E 10000mAh.
It worked perfectly, just as he said.
I charged it up completely when I rec'd and plugged my Nook HD+ into it. The Nook lit up and said it was charging which my last battery extender never did. After a few hours my Nook was 100% charged and the extender was down to about 75%. I then recharged the extender in a couple of hours.
I'm in the market for second battery extender, and that one's more powerful than mine, so maybe I'll check it out. Currently, I only have one, which I use for my NOOK HD and for my Samsung Galaxy phone: the I/O Magic Universal Rechargeable USB Battery Power Pack (4400mAh). It charges my devices quickly, holds a charge well, recharges quickly, and is a "smart charging" device that knows when to stop imbibing "juice" when it's fully charged, even if it remains plugged in. Three blue lights on its front panel indicate its battery charge level.
Mophie charging packs, and anything else rated to charge an ipad, should work. The key is to get a 2-amp charging port.
Quite a few of the battery packs sold on eBay do not work with a Nook HD+. It makes no difference if they have a 2A port or not. I think what turnsouth posted is what you have to look for.
The 1st battery pack I bought had a 2A port and it would not work with my Nook, yet it would charge either of my SP's with no trouble at all.
Well, as Wulfraed mentioned, all proper devices are supposed to do that. If they're not handling the connection properly, that's (theoretically) a separate issue. But to be on the safe side, the OPs best bet is to get the Anker device mentioned above - it's a tried and true device.